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SiO2 interfaces
Materials Science and Engineering B 154–155 (2008) 264–267
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Segregation of antimony to Si/SiO2 interfaces C. Steen a,1 , P. Pichler a,b,∗ , H. Ryssel a,b a b
Chair of Electron Devices, University of Erlangen-Nuremberg, Cauerstrasse 6, 91058 Erlangen, Germany Fraunhofer-Institute of Integrated Systems and Device Technology, Schottkystrasse 10, 91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 5 May 2008 Received in revised form 8 July 2008 Accepted 12 August 2008 Keywords: Antimony Silicon Surface segregation TXRF
a b s t r a c t Segregation of n-type dopants to interfaces is an important contribution to the loss of electrical activity in current and future device generations. In this work, the segregation and pile-up of antimony atoms at the Si/SiO2 interface was investigated at a temperature of 1000 ◦ C by a combination of gracing incidence X-ray fluorescence spectroscopy (GI-XRF) measurements, electrical measurements, and etching on the nanometer scale. Long annealing times were used to make sure that the segregated atoms are in steady state with the antimony atoms in the bulk. Assuming that the segregated atoms are electrically neutral, their sheet concentration can be modeled as steady state with positively charged substitutional antimony atoms and free electrons. © 2008 Elsevier B.V. All rights reserved.
1. Introduction For the continuous shrinking of the dimensions of semiconductor devices, shallower and shallower source/drain profiles with higher and higher electrical activation are required. In the quest for possible replacements for arsenic and phosphorus as predominant doping elements for n-channel devices, antimony has invoked renewed interests. Although its equilibrium solubility is an order of magnitude smaller than that of arsenic [1], there are indications that significantly higher electrical activation can be achieved in constrained environments or under metastable conditions. Citrin et al. [2] reported electrically active concentrations of up to 5 × 1020 cm−3 in a two-dimensional arrangement. They suggested that it is possible to inhibit the formation of inactive complexes by geometrical constraints. In the work of Bennett et al. [3] it was shown that the solubility of antimony increases significantly under tensile strain. Finally, in the work of Williams [4] and Duffy et al. [5], electrically active antimony concentrations on the order of 1021 cm−3 were reported after solid-phase epitaxial regrowth (SPER). In addition, since antimony diffuses nearly entirely via a vacancy mechanism [6], no transient enhanced diffusion phenomena were found to occur during post-implantation annealing [7,8].
∗ Corresponding author at: Fraunhofer-Institute of Integrated Systems and Device Technology, Schottkystrasse 10, 91058 Erlangen, Germany.Tel.: +49 9131 761 227; fax: +49 9131 761 212. E-mail address: [email protected] (P. Pichler). 1 Present address: Lineas Automotive GmbH, Nuremberg, Germany. 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.08.011
Since dopant profiles became shallower and shallower, the segregation and pile-up of dopants at the interface Si/SiO2 is now one of the major limiting factors for the activation of common n-type dopants. This phenomenon was well studied for phosphorus and arsenic and the interested reader is referred to the publication of Dabrowski et al. [9] and to our recent work [10], respectively, as well as to the literature cited therein. For antimony, similar evidence was reported in the literature. The investigation of Sai-Halasz et al. [11] indicated that up to 40% of the implanted antimony dose of 7 × 1014 cm−2 piled-up at the silicon side of the interface and became electrically inactive after annealing. Similarly, in the work of Shibahara et al. [12,13], the activation of shallow antimony implants was found to be decreased significantly by the pile-up phenomenon. They also reported that the piled-up antimony can be removed by an HF dip. Varying the implantation dose, Shibahara et al. [13] came to the conclusion that the pile-up is caused by SPER if the samples are amorphized. A shift of the implanted antimony atoms due to SPER was reported also by Collart et al. [7] on the basis of medium-energy ion scattering measurements. Using timeof-flight secondary ion mass spectrometry (TOF-SIMS), Krüger et al. [8] concluded that the antimony atoms reside on the silicon side of the interface. Based on TEM measurements, Shibahara [14] and Krüger et al. [8] excluded that the observed pile-up is associated with macroscopic antimony precipitates. Although the pile-up of antimony at the interface between silicon and silicon dioxide is well documented and qualitatively very similar to the behavior of phosphorus and arsenic, there is nearly no quantitative information available that could be used for process simulation. Our approach to characterize the sheet concentration of antimony atoms segregated at the interface is based on gracing
C. Steen et al. / Materials Science and Engineering B 154–155 (2008) 264–267
265
incidence X-ray fluorescence spectroscopy (GI-XRF) in combination with etching of the silicon in analogy to our previous work on arsenic [10,15]. Various samples doped with different doses and annealed at 1000 ◦ C were investigated. Based on the experimental data obtained, we propose a model describing the relation between the sheet concentration of segregated antimony atoms and the concentration of substitutional atoms. 2. Experimental details All experiments were performed with p-type 100 mm Czochralski (1 0 0) Si wafers (4–6 cm). Before the implantation of antimony, the native oxide was removed by an HF dip to avoid oxygen knock-on. Antimony was then implanted with an energy of 10 keV and doses from 1 × 1014 to 3 × 1015 cm−2 , or with an energy of 50 keV and doses from 3 × 1014 to 3 × 1015 cm−2 . After the implantation, the wafers were cleaned with a 5:1 (vol) mixture of 96% H2 SO4 and 30% H2 O2 at 140 ◦ C for 20 min, followed by an HF dip, and finally a rinse in an aqueous solution of choline (trimethyl-2hydroxyethyl ammonium hydroxide) acting as a complexing agent for metal impurities. Thereafter, thermal oxides were grown at 800 ◦ C in dry O2 with 3% dichloroethylene. The processes resulted in oxide thicknesses of 4.6–6.5 nm, depending on the implanted dose. After the oxidation the wafers were annealed in a horizontal furnace in an N2 ambient at 1000 ◦ C for periods from 20 to 180 min. Some samples implanted with 50 keV received an additional anneal at 1000 ◦ C for 220 or 280 min since it was found that the first annealing was not sufficient to establish steady state between the segregated atoms and the substitutional atoms in the bulk. For all annealings, the ramp rates were 5 K/min during ramping up and −2 K/min during cooling down. Due to annealing, the oxide thickness increased by about 1–2 nm. The GI-XRF measurements were performed using an ATOMIKA 8300W. The excitation source was a long fine focus X-ray tube with a tungsten anode operated at 50 kV and 20 mA. The fluorescence signal was measured as a function of the glancing angle. At large glancing angles, the fluorescence signal is a measure for the total sheet concentration of dopant atoms in the sample since at these angles the penetration depth of the X-ray beam becomes deep enough to excite all antimony atoms with almost the same X-ray intensity. All simulations of the angle dependence of the fluorescence signal were based on the model of Weisbrod et al. [16]. Covering oxide layers as well as the silicon layer containing the pile-up region were removed in the present work by immersing the samples into HF. Thicker silicon layers in the bulk were removed by anodic oxidation and removal of the grown oxide by diluted HF [17]. The electrolyte used for the anodic oxidation was 0.1 M HCl. A constant current density of 0.32 mA/cm2 was applied for 90 s between the sample and a Pt-anode, leading to the growth of thin oxides with thicknesses of a few nm. The oxides were removed with diluted HF (1%). This procedure was repeated in order to remove thicker layers. The anodic oxidation was performed locally with an apparatus similar to that described by Ryssel et al. [18]. The thickness of the removed silicon layer was determined by measuring the step height with a Micromap-512 interferometry profilometer with a 10× objective operated in wave mode. The method is applicable for step heights exceeding about 5 nm. Four-point probe (4PP) measurements were performed to obtain the electrically active concentration below the pile-up layer by using a Prometrix OmniMap RS50/e. Ellipsometer measurements of oxide thicknesses were performed using a PLASMOS EST 2/3 at a wavelength of 632.8 nm, an incident angle of 70◦ , and an angle of the polarizer of 45◦ .
Fig. 1. Fluorescence signal as a function of the incidence angle for a sample implanted with 1 × 1015 cm−2 and annealed for 180 + 220 min at 1000 ◦ C. The symbols refer to different etching procedures prior to the GI-XRF measurements, the lines are the respective simulations for the final estimate of the antimony profile.
3. Results and discussion Since we found that the angular dependence of the fluorescence signal in GI-XRF measurements is not sufficient to extract depth profiles and segregated sheet concentrations unambiguously [15], such measurements were combined with etching processes on the nanometer scale. Fig. 1 shows as an example the results of GI-XRF measurements on a sample implanted with antimony at an energy of 50 keV and a dose of 1 × 1015 cm−2 which was annealed successively in a furnace for 180 and 220 min at 1000 ◦ C. The symbols refer to the various etching procedures discussed below that were applied before the GI-XRF measurements, the lines to the respective simulations. All etching procedures were performed locally on the same wafer within a circle of 2 cm diameter. In a first step, the oxide and the pile-up region were removed. After an etching of 5 min using diluted HF (1%), the surface became hydrophobic, indicating the removal of the oxide layer. In this step, the sheet concentration of antimony in the sample was reduced considerably. A larger reduction was found for etching times of 10 min. In contrast, no significant additional removal of antimony was found when the etching time was extended from 10 to 15 min. This was taken as criterion for the complete removal of the pile-up region and the respective GI-XRF curve is shown in Fig. 1. It was not possible to characterize the samples implanted with 3 × 1015 cm−2 with this method. The respective 10 keV samples never became hydrophobic although step height measurements indicated that more than 20 nm of silicon was already removed. In contrast, for the respective 50 keV samples, soaking the samples in diluted HF (1%) or concentrated HF (50%) for even longer than 15 min did not lead to any significant reduction of the antimony sheet concentration. After removal of the piled-up antimony, thicker silicon layers had to be removed to measure a significant antimony dose loss with GI-XRF. As described above, this was achieved by anodic oxidation and removal of these oxide layers with diluted HF. Since the total thicknesses of the layers removed exceeded 5 nm, they could be characterized directly by an interferometry profilometer. This allowed calculating the bulk concentration as quotient of the sheet concentration removed and the thickness of the removed layer.
266
C. Steen et al. / Materials Science and Engineering B 154–155 (2008) 264–267
power of 1.6. This is similar to the behavior reported for phosphorus [9] and arsenic [10]. For both, the relation between the sheet concentration of segregated atoms and the concentration of substitutional atoms was found to have linear and quadratic components. Unlike for phosphorus and arsenic, no saturation of the segregated sheet concentration was observed for antimony. This is probably related to the limitation of the substitutional concentration by the solid solubility. An extension of the GI-XRF investigation towards smaller bulk concentrations was not possible because of the high detection limit of GI-XRF for antimony due to inherent limitations of the method. 4. Modeling the pile-up of antimony Assuming that the segregated antimony atoms Sb0seg are electrically inactive, they can be thought to result from the reaction 0 − Sb+ s + e Sbseg
Fig. 2. Sheet concentration of piled-up antimony as a function of the electrically active antimony concentration below the pile-up region. The full line was calculated with the model presented in Section 4. Also shown are the corresponding curves for phosphorus [9] and arsenic [10].
(2)
between positively charged substitutional antimony atoms Sb+ s and free electrons e− . In steady state, the sheet concentration NSb0 of seg
segregated antimony atoms in the pile-up region is then proportional to the product NSb0
seg
=
t n C + e Sbs ni
(3)
The segregated atoms in the pile-up region can be assumed to be in steady state with the electrically active substitutional atoms. From the GI-XRF measurements in combination with anodic oxidation one obtains the total antimony concentration in the bulk below the segregation layer. For the highest implanted doses, the total concentration comprises substitutional antimony atoms and antimony atoms in inactive complexes, presumably antimony-vacancy complexes [19]. To obtain the concentration of substitutional antimony atoms CSb+ , complementary four-point probe sheet resis-
of the substitutional concentration of antimony atoms CSb+ and
tance measurements were performed before and after the anodic oxidation. From them, CSb+ was calculated as a self-consistent solu-
For substitutional antimony concentrations below ni , the electron concentration equals approximately n ≈ ni and the concentration of segregated atoms (3) increases linearly with the substitutional antimony concentration. When CSb+ exceeds the intrinsic carrier
s
the electron concentration n. t and e are parameters governing the emission from and trapping to the pile-up region. The electron concentration can be approximately calculated from the assumption of charge neutrality and is then given by n=
CSb+ s
2
+
CSb+ s
2
2 + ni 2 .
(4)
s
s
tion of
qCSb+ CSb+ s
s
t =
1 Rs1
−
1 Rs2
(1)
with q denoting the elementary charge, t the thickness of the removed layer, and Rs1 and Rs2 the sheet resistances before and after layer For the concentration dependence of the mobility removal. CSb+ , the model of Masetti et al. [20] was used. It was found s
that the active and total concentrations agreed only for doses below 1015 cm−2 . As an example, for the sample implanted with 50 keV and a dose of 3 × 1015 cm−2 annealed in succession for 180 and 220 min, the total antimony concentration was found to be with 1.2 × 1020 cm−3 an order of magnitude larger than the electrically active concentration of 1.5 × 1019 cm−3 . The relationship between the sheet concentration of piled-up antimony atoms and the background concentration of substitutional, active antimony atoms is shown in Fig. 2. A comparison of the data obtained after implantation with an energy of 10 keV and annealing for 20 or 60 min led to the conclusion that the former annealing was probably not long enough to establish steady state between the segregated antimony atoms and the substitutional ones. Similar discrepancies were observed for the 50 keV implants after annealing for 20 and 180 min. Therefore, some samples were additionally annealed for 220 min. Thereafter, the measurements indicated the establishment of the desired steady state. The data points representing steady state conditions between segregated and substitutional antimony atoms are on a curve which increases approximately with the background concentration to a
s
concentration significantly, the electron concentration approaches the concentration of substitutional atoms n ≈ CSb+ so that the cons
centration of segregated atoms (3) increases quadratically with the substitutional antimony concentration. For an implementation into existing process simulation programs, the three-phase model suggested first by Lau et al. [21] and Orlowski [22] can be adapted. It considers trapping of atoms to and emission from an interface layer with a maximum sheet concenmax of sites that can be occupied. Although it is unclear tration NSb at present whether such a saturation exists for antimony, our data max should be significantly larger than 1015 cm−2 . suggest that NSb The line in Fig. 2 was obtained with t/e = 1.9 × 10−5 cm and ni (9.5 × 1018 cm−3 at 1000 ◦ C) taken from Morin and Maita [23]. Also shown are the model predictions from the work of Dabrowski et al. [9] for phosphorus and of our previous work [10] for arsenic. It is interesting to note that the sheet concentration of segregated antimony atoms is by a factor of about 8 larger than that of arsenic for the same substitutional concentration. 5. Conclusion GI-XRF and electrical measurements were combined with successive etching of silicon layers to characterize the steady state of substitutional antimony atoms and antimony atoms segregated to the interface between silicon and silicon dioxide at 1000 ◦ C. It was found that the sheet concentration of segregated antimony atoms
C. Steen et al. / Materials Science and Engineering B 154–155 (2008) 264–267
exceeds the respective value for arsenic by a factor of about 8 for the same substitutional concentration. A quantitative model was proposed to describe the experiments.
[8] [9]
Acknowledgments This work was funded by the Deutsche Forschungsgemeinschaft under contract no. Ry 1/23. The authors would also like to thank their colleagues from the technology department for performing the implants and anneals. References [1] R. Angelucci, A. Armigliato, E. Landi, D. Nobili, S. Solmi, in: ESSDERC 87, Tecnoprint, Bologna, 1987, p. 461. [2] P.H. Citrin, D.A. Muller, H.-J. Gossmann, R. Vanfleet, P.A. Northrup, Phys. Rev. Lett. 83 (1999) 3234. [3] N.S. Bennett, A.J. Smith, R.M. Gwilliam, R.P. Webb, B.J. Sealy, N.E.B. Cowern, L. O’Reilly, P.J. McNally, J. Vac. Sci. Technol. B 26 (2008) 391. [4] J.S. Williams, Nucl. Instrum. Methods 209–210 (1983) 219. [5] R. Duffy, T. Dao, Y. Tamminga, K. van der Tak, F. Roozeboom, E. Augendre, Appl. Phys. Lett. 89 (2006) 071915. [6] T.Y. Tan, B.J. Ginsberg, Appl. Phys. Lett. 42 (1983) 448. [7] E.J.H. Collart, D. Kirkwood, J.A. Van den Berg, M. Werner, W. Vandervorst, B. Brijs, P. Bailey, T.C.Q. Noakes, in: B. Brown, T.L. Alford, M. Nastasi, M.C. Vella
[10] [11] [12]
[13]
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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(Eds.), 2002 14th International Conference on Ion Implantation Technology, IEEE, Piscataway, 2003, p. 147. D. Krüger, H. Rücker, B. Heinemann, V. Melnik, R. Kurps, D. Bolze, J. Vac. Sci. Technol. B 22 (2004) 455. J. Dabrowski, H.-J. Müssig, V. Zavodinsky, R. Baierle, M.J. Caldas, Phys. Rev. B 65 (2002) 245305. C. Steen, A. Martinez-Limia, P. Pichler, H. Ryssel, S. Paul, W. Lerch, L. Pei, G. Duscher, F. Severac, F. Cristiano, W. Windl, J. Appl. Phys. 104 (2008) 023518. G.A. Sai-Halasz, K.T. Short, J.S. Williams, IEEE Electron. Device Lett. EDL-6 (1985) 285. K. Shibahara, N. Mifuji, K. Kawabata, T. Kugimiya, H. Furumoto, M. Tauno, S. Yokoyama, M. Nagata, S. Miyazaki, M. Hirose, Technical Digest of the 1996 International Electron Devices Meeting (IEDM), IEEE, Piscataway, 1996, p. 579. K. Shibahara, H. Furumoto, K. Egusa, M. Koh, S. Yokoyama, in: N.E.B. Cowern, D.C. Jacobson, P.B. Griffin, P.A. Packan, R.P. Webb (Eds.), Silicon Front-end Technology—Materials Processing and Modelling, Mater. Res. Soc. Symp. Proc., vol. 532 1998, p. 23. K. Shibahara, IEICE Trans. Electron. E85-C (2002) 1091. C. Steen, A. Nutsch, P. Pichler, H. Ryssel, Spectrochim. Acta B 62 (2007) 481. U. Weisbrod, R. Gutschke, J. Knoth, H. Schwenke, Appl. Phys. A 53 (1991) 449. J.A. Bardwell, N. Draper, P. Schmuki, J. Appl. Phys. 79 (1996) 8761. H. Ryssel, K. Schmid, H. Müller, J. Phys. E 6 (1973) 492. A. Nylandsted Larsen, F.T. Pedersen, G. Weyer, R. Galloni, R. Rizzoli, A. Armigliato, J. Appl. Phys. 59 (1986) 1908. G. Masetti, M. Severi, S. Solmi, IEEE Trans. Electron. Dev. ED-30 (1983) 764. F. Lau, L. Mader, C. Mazure, C. Werner, M. Orlowski, Appl. Phys. A 49 (1989) 671. M. Orlowski, Appl. Phys. Lett. 55 (1989) 1762. F.J. Morin, J.P. Maita, Phys. Rev. 96 (1954) 28.
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Segregation of antimony to Si/SiO2 interfaces C. Steen a,1 , P. Pichler a,b,∗ , H. Ryssel a,b a b
Chair of Electron Devices, University of Erlangen-Nuremberg, Cauerstrasse 6, 91058 Erlangen, Germany Fraunhofer-Institute of Integrated Systems and Device Technology, Schottkystrasse 10, 91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 5 May 2008 Received in revised form 8 July 2008 Accepted 12 August 2008 Keywords: Antimony Silicon Surface segregation TXRF
a b s t r a c t Segregation of n-type dopants to interfaces is an important contribution to the loss of electrical activity in current and future device generations. In this work, the segregation and pile-up of antimony atoms at the Si/SiO2 interface was investigated at a temperature of 1000 ◦ C by a combination of gracing incidence X-ray fluorescence spectroscopy (GI-XRF) measurements, electrical measurements, and etching on the nanometer scale. Long annealing times were used to make sure that the segregated atoms are in steady state with the antimony atoms in the bulk. Assuming that the segregated atoms are electrically neutral, their sheet concentration can be modeled as steady state with positively charged substitutional antimony atoms and free electrons. © 2008 Elsevier B.V. All rights reserved.
1. Introduction For the continuous shrinking of the dimensions of semiconductor devices, shallower and shallower source/drain profiles with higher and higher electrical activation are required. In the quest for possible replacements for arsenic and phosphorus as predominant doping elements for n-channel devices, antimony has invoked renewed interests. Although its equilibrium solubility is an order of magnitude smaller than that of arsenic [1], there are indications that significantly higher electrical activation can be achieved in constrained environments or under metastable conditions. Citrin et al. [2] reported electrically active concentrations of up to 5 × 1020 cm−3 in a two-dimensional arrangement. They suggested that it is possible to inhibit the formation of inactive complexes by geometrical constraints. In the work of Bennett et al. [3] it was shown that the solubility of antimony increases significantly under tensile strain. Finally, in the work of Williams [4] and Duffy et al. [5], electrically active antimony concentrations on the order of 1021 cm−3 were reported after solid-phase epitaxial regrowth (SPER). In addition, since antimony diffuses nearly entirely via a vacancy mechanism [6], no transient enhanced diffusion phenomena were found to occur during post-implantation annealing [7,8].
∗ Corresponding author at: Fraunhofer-Institute of Integrated Systems and Device Technology, Schottkystrasse 10, 91058 Erlangen, Germany.Tel.: +49 9131 761 227; fax: +49 9131 761 212. E-mail address: [email protected] (P. Pichler). 1 Present address: Lineas Automotive GmbH, Nuremberg, Germany. 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.08.011
Since dopant profiles became shallower and shallower, the segregation and pile-up of dopants at the interface Si/SiO2 is now one of the major limiting factors for the activation of common n-type dopants. This phenomenon was well studied for phosphorus and arsenic and the interested reader is referred to the publication of Dabrowski et al. [9] and to our recent work [10], respectively, as well as to the literature cited therein. For antimony, similar evidence was reported in the literature. The investigation of Sai-Halasz et al. [11] indicated that up to 40% of the implanted antimony dose of 7 × 1014 cm−2 piled-up at the silicon side of the interface and became electrically inactive after annealing. Similarly, in the work of Shibahara et al. [12,13], the activation of shallow antimony implants was found to be decreased significantly by the pile-up phenomenon. They also reported that the piled-up antimony can be removed by an HF dip. Varying the implantation dose, Shibahara et al. [13] came to the conclusion that the pile-up is caused by SPER if the samples are amorphized. A shift of the implanted antimony atoms due to SPER was reported also by Collart et al. [7] on the basis of medium-energy ion scattering measurements. Using timeof-flight secondary ion mass spectrometry (TOF-SIMS), Krüger et al. [8] concluded that the antimony atoms reside on the silicon side of the interface. Based on TEM measurements, Shibahara [14] and Krüger et al. [8] excluded that the observed pile-up is associated with macroscopic antimony precipitates. Although the pile-up of antimony at the interface between silicon and silicon dioxide is well documented and qualitatively very similar to the behavior of phosphorus and arsenic, there is nearly no quantitative information available that could be used for process simulation. Our approach to characterize the sheet concentration of antimony atoms segregated at the interface is based on gracing
C. Steen et al. / Materials Science and Engineering B 154–155 (2008) 264–267
265
incidence X-ray fluorescence spectroscopy (GI-XRF) in combination with etching of the silicon in analogy to our previous work on arsenic [10,15]. Various samples doped with different doses and annealed at 1000 ◦ C were investigated. Based on the experimental data obtained, we propose a model describing the relation between the sheet concentration of segregated antimony atoms and the concentration of substitutional atoms. 2. Experimental details All experiments were performed with p-type 100 mm Czochralski (1 0 0) Si wafers (4–6 cm). Before the implantation of antimony, the native oxide was removed by an HF dip to avoid oxygen knock-on. Antimony was then implanted with an energy of 10 keV and doses from 1 × 1014 to 3 × 1015 cm−2 , or with an energy of 50 keV and doses from 3 × 1014 to 3 × 1015 cm−2 . After the implantation, the wafers were cleaned with a 5:1 (vol) mixture of 96% H2 SO4 and 30% H2 O2 at 140 ◦ C for 20 min, followed by an HF dip, and finally a rinse in an aqueous solution of choline (trimethyl-2hydroxyethyl ammonium hydroxide) acting as a complexing agent for metal impurities. Thereafter, thermal oxides were grown at 800 ◦ C in dry O2 with 3% dichloroethylene. The processes resulted in oxide thicknesses of 4.6–6.5 nm, depending on the implanted dose. After the oxidation the wafers were annealed in a horizontal furnace in an N2 ambient at 1000 ◦ C for periods from 20 to 180 min. Some samples implanted with 50 keV received an additional anneal at 1000 ◦ C for 220 or 280 min since it was found that the first annealing was not sufficient to establish steady state between the segregated atoms and the substitutional atoms in the bulk. For all annealings, the ramp rates were 5 K/min during ramping up and −2 K/min during cooling down. Due to annealing, the oxide thickness increased by about 1–2 nm. The GI-XRF measurements were performed using an ATOMIKA 8300W. The excitation source was a long fine focus X-ray tube with a tungsten anode operated at 50 kV and 20 mA. The fluorescence signal was measured as a function of the glancing angle. At large glancing angles, the fluorescence signal is a measure for the total sheet concentration of dopant atoms in the sample since at these angles the penetration depth of the X-ray beam becomes deep enough to excite all antimony atoms with almost the same X-ray intensity. All simulations of the angle dependence of the fluorescence signal were based on the model of Weisbrod et al. [16]. Covering oxide layers as well as the silicon layer containing the pile-up region were removed in the present work by immersing the samples into HF. Thicker silicon layers in the bulk were removed by anodic oxidation and removal of the grown oxide by diluted HF [17]. The electrolyte used for the anodic oxidation was 0.1 M HCl. A constant current density of 0.32 mA/cm2 was applied for 90 s between the sample and a Pt-anode, leading to the growth of thin oxides with thicknesses of a few nm. The oxides were removed with diluted HF (1%). This procedure was repeated in order to remove thicker layers. The anodic oxidation was performed locally with an apparatus similar to that described by Ryssel et al. [18]. The thickness of the removed silicon layer was determined by measuring the step height with a Micromap-512 interferometry profilometer with a 10× objective operated in wave mode. The method is applicable for step heights exceeding about 5 nm. Four-point probe (4PP) measurements were performed to obtain the electrically active concentration below the pile-up layer by using a Prometrix OmniMap RS50/e. Ellipsometer measurements of oxide thicknesses were performed using a PLASMOS EST 2/3 at a wavelength of 632.8 nm, an incident angle of 70◦ , and an angle of the polarizer of 45◦ .
Fig. 1. Fluorescence signal as a function of the incidence angle for a sample implanted with 1 × 1015 cm−2 and annealed for 180 + 220 min at 1000 ◦ C. The symbols refer to different etching procedures prior to the GI-XRF measurements, the lines are the respective simulations for the final estimate of the antimony profile.
3. Results and discussion Since we found that the angular dependence of the fluorescence signal in GI-XRF measurements is not sufficient to extract depth profiles and segregated sheet concentrations unambiguously [15], such measurements were combined with etching processes on the nanometer scale. Fig. 1 shows as an example the results of GI-XRF measurements on a sample implanted with antimony at an energy of 50 keV and a dose of 1 × 1015 cm−2 which was annealed successively in a furnace for 180 and 220 min at 1000 ◦ C. The symbols refer to the various etching procedures discussed below that were applied before the GI-XRF measurements, the lines to the respective simulations. All etching procedures were performed locally on the same wafer within a circle of 2 cm diameter. In a first step, the oxide and the pile-up region were removed. After an etching of 5 min using diluted HF (1%), the surface became hydrophobic, indicating the removal of the oxide layer. In this step, the sheet concentration of antimony in the sample was reduced considerably. A larger reduction was found for etching times of 10 min. In contrast, no significant additional removal of antimony was found when the etching time was extended from 10 to 15 min. This was taken as criterion for the complete removal of the pile-up region and the respective GI-XRF curve is shown in Fig. 1. It was not possible to characterize the samples implanted with 3 × 1015 cm−2 with this method. The respective 10 keV samples never became hydrophobic although step height measurements indicated that more than 20 nm of silicon was already removed. In contrast, for the respective 50 keV samples, soaking the samples in diluted HF (1%) or concentrated HF (50%) for even longer than 15 min did not lead to any significant reduction of the antimony sheet concentration. After removal of the piled-up antimony, thicker silicon layers had to be removed to measure a significant antimony dose loss with GI-XRF. As described above, this was achieved by anodic oxidation and removal of these oxide layers with diluted HF. Since the total thicknesses of the layers removed exceeded 5 nm, they could be characterized directly by an interferometry profilometer. This allowed calculating the bulk concentration as quotient of the sheet concentration removed and the thickness of the removed layer.
266
C. Steen et al. / Materials Science and Engineering B 154–155 (2008) 264–267
power of 1.6. This is similar to the behavior reported for phosphorus [9] and arsenic [10]. For both, the relation between the sheet concentration of segregated atoms and the concentration of substitutional atoms was found to have linear and quadratic components. Unlike for phosphorus and arsenic, no saturation of the segregated sheet concentration was observed for antimony. This is probably related to the limitation of the substitutional concentration by the solid solubility. An extension of the GI-XRF investigation towards smaller bulk concentrations was not possible because of the high detection limit of GI-XRF for antimony due to inherent limitations of the method. 4. Modeling the pile-up of antimony Assuming that the segregated antimony atoms Sb0seg are electrically inactive, they can be thought to result from the reaction 0 − Sb+ s + e Sbseg
Fig. 2. Sheet concentration of piled-up antimony as a function of the electrically active antimony concentration below the pile-up region. The full line was calculated with the model presented in Section 4. Also shown are the corresponding curves for phosphorus [9] and arsenic [10].
(2)
between positively charged substitutional antimony atoms Sb+ s and free electrons e− . In steady state, the sheet concentration NSb0 of seg
segregated antimony atoms in the pile-up region is then proportional to the product NSb0
seg
=
t n C + e Sbs ni
(3)
The segregated atoms in the pile-up region can be assumed to be in steady state with the electrically active substitutional atoms. From the GI-XRF measurements in combination with anodic oxidation one obtains the total antimony concentration in the bulk below the segregation layer. For the highest implanted doses, the total concentration comprises substitutional antimony atoms and antimony atoms in inactive complexes, presumably antimony-vacancy complexes [19]. To obtain the concentration of substitutional antimony atoms CSb+ , complementary four-point probe sheet resis-
of the substitutional concentration of antimony atoms CSb+ and
tance measurements were performed before and after the anodic oxidation. From them, CSb+ was calculated as a self-consistent solu-
For substitutional antimony concentrations below ni , the electron concentration equals approximately n ≈ ni and the concentration of segregated atoms (3) increases linearly with the substitutional antimony concentration. When CSb+ exceeds the intrinsic carrier
s
the electron concentration n. t and e are parameters governing the emission from and trapping to the pile-up region. The electron concentration can be approximately calculated from the assumption of charge neutrality and is then given by n=
CSb+ s
2
+
CSb+ s
2
2 + ni 2 .
(4)
s
s
tion of
qCSb+ CSb+ s
s
t =
1 Rs1
−
1 Rs2
(1)
with q denoting the elementary charge, t the thickness of the removed layer, and Rs1 and Rs2 the sheet resistances before and after layer For the concentration dependence of the mobility removal. CSb+ , the model of Masetti et al. [20] was used. It was found s
that the active and total concentrations agreed only for doses below 1015 cm−2 . As an example, for the sample implanted with 50 keV and a dose of 3 × 1015 cm−2 annealed in succession for 180 and 220 min, the total antimony concentration was found to be with 1.2 × 1020 cm−3 an order of magnitude larger than the electrically active concentration of 1.5 × 1019 cm−3 . The relationship between the sheet concentration of piled-up antimony atoms and the background concentration of substitutional, active antimony atoms is shown in Fig. 2. A comparison of the data obtained after implantation with an energy of 10 keV and annealing for 20 or 60 min led to the conclusion that the former annealing was probably not long enough to establish steady state between the segregated antimony atoms and the substitutional ones. Similar discrepancies were observed for the 50 keV implants after annealing for 20 and 180 min. Therefore, some samples were additionally annealed for 220 min. Thereafter, the measurements indicated the establishment of the desired steady state. The data points representing steady state conditions between segregated and substitutional antimony atoms are on a curve which increases approximately with the background concentration to a
s
concentration significantly, the electron concentration approaches the concentration of substitutional atoms n ≈ CSb+ so that the cons
centration of segregated atoms (3) increases quadratically with the substitutional antimony concentration. For an implementation into existing process simulation programs, the three-phase model suggested first by Lau et al. [21] and Orlowski [22] can be adapted. It considers trapping of atoms to and emission from an interface layer with a maximum sheet concenmax of sites that can be occupied. Although it is unclear tration NSb at present whether such a saturation exists for antimony, our data max should be significantly larger than 1015 cm−2 . suggest that NSb The line in Fig. 2 was obtained with t/e = 1.9 × 10−5 cm and ni (9.5 × 1018 cm−3 at 1000 ◦ C) taken from Morin and Maita [23]. Also shown are the model predictions from the work of Dabrowski et al. [9] for phosphorus and of our previous work [10] for arsenic. It is interesting to note that the sheet concentration of segregated antimony atoms is by a factor of about 8 larger than that of arsenic for the same substitutional concentration. 5. Conclusion GI-XRF and electrical measurements were combined with successive etching of silicon layers to characterize the steady state of substitutional antimony atoms and antimony atoms segregated to the interface between silicon and silicon dioxide at 1000 ◦ C. It was found that the sheet concentration of segregated antimony atoms
C. Steen et al. / Materials Science and Engineering B 154–155 (2008) 264–267
exceeds the respective value for arsenic by a factor of about 8 for the same substitutional concentration. A quantitative model was proposed to describe the experiments.
[8] [9]
Acknowledgments This work was funded by the Deutsche Forschungsgemeinschaft under contract no. Ry 1/23. The authors would also like to thank their colleagues from the technology department for performing the implants and anneals. References [1] R. Angelucci, A. Armigliato, E. Landi, D. Nobili, S. Solmi, in: ESSDERC 87, Tecnoprint, Bologna, 1987, p. 461. [2] P.H. Citrin, D.A. Muller, H.-J. Gossmann, R. Vanfleet, P.A. Northrup, Phys. Rev. Lett. 83 (1999) 3234. [3] N.S. Bennett, A.J. Smith, R.M. Gwilliam, R.P. Webb, B.J. Sealy, N.E.B. Cowern, L. O’Reilly, P.J. McNally, J. Vac. Sci. Technol. B 26 (2008) 391. [4] J.S. Williams, Nucl. Instrum. Methods 209–210 (1983) 219. [5] R. Duffy, T. Dao, Y. Tamminga, K. van der Tak, F. Roozeboom, E. Augendre, Appl. Phys. Lett. 89 (2006) 071915. [6] T.Y. Tan, B.J. Ginsberg, Appl. Phys. Lett. 42 (1983) 448. [7] E.J.H. Collart, D. Kirkwood, J.A. Van den Berg, M. Werner, W. Vandervorst, B. Brijs, P. Bailey, T.C.Q. Noakes, in: B. Brown, T.L. Alford, M. Nastasi, M.C. Vella
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